1. Field of the Invention
The present invention relates to a fault diagnosis apparatus for a fuel evaporative emission suppressing system.
2. Description of the Related Art
In order to prevent air pollution and the like, the engine and body of an automobile are provided with various devices for treating harmful emission components. These known devices include, for example, a blow-by gas recirculating device for guiding a blow-by gas, which consists mainly of an unburned fuel component (HC) leaking from a combustion chamber of the engine into a crank case, to an intake pipe, and a fuel evaporative emission suppressing device for guiding a fuel evaporative gas, composed mainly of HC produced in a fuel tank, into the intake pipe.
The fuel evaporative emission suppressing device comprises a canister, loaded with activated charcoal which adsorbs the fuel evaporative gas, a large number pipes, etc. The canister is provided with an inlet port, outlet port, and vent port which open into the fuel tank, intake pipe, and atmosphere, respectively. In the fuel evaporative emission suppressing device of this canister-storage type, the fuel evaporative gas in the fuel tank is introduced into the canister and made to be adsorbed by the activated charcoal. The atmosphere (purge air) is introduced into the canister through the vent port by applying a negative pressure in the intake pipe to the outlet port. The fuel evaporative gas adsorbed by the activated charcoal is separated therefrom by means of the purge air, and the separated gas, along with the purge air, is introduced into the intake pipe. The fuel evaporative gas, thus delivered into the intake pipe, is burned in the combustion chamber of the engine, whereby it is prevented from being discharged into the atmosphere.
If the purge air containing the fuel evaporative gas is introduced carelessly into the intake pipe, however, the air-fuel ratio of an air-fuel mixture deviates from its appropriate range, so that the rotational speed and output torque of the engine fluctuate greatly. Accordingly, the comfortableness to drive or drivability of the vehicle worsens. This unfavorable phenomenon is particularly remarkable in the case where the purge air is introduced while the engine is running in an idling area in which the quantity of intake air is small.
To avoid this, a purge control valve, for use as purge regulating means for controlling the rate of purge air introduction, is provided in a purge passage which connects the canister and the intake pipe. The purge control valve is opened to allow the purge air to be introduced into the engine only when the engine is operating in a predetermined operation area. In general, purge control valves may be classified into two types, mechanical ones which operate in response to negative intake pressure and electrical ones which are controlled in on-off operation by means of an electronic control unit (ECU) in accordance with pieces of operation information, such as the throttle opening, intake air flow rate, etc. Although the mechanical valves, low-priced, are widely used, the electrical or solenoid-operated valves are superior in performance, since the introduction and shut-off of the purge air can be controlled more accurately and freely by the electrical ones.
In the fuel evaporative emission suppressing device furnished with one such solenoid-operated purge control valve, however, snapping of wires which connect the ECU and the purge control valve, connector contact failure, etc. may occur, or a valve plug in the control valve may possibly be fixed in a closed state from some cause. In such a case, the purge air cannot be introduced into the intake pipe, so that the canister is overloaded with the fuel evaporative gas. Inevitably, therefore, the fuel evaporative gas additionally supplied from the fuel tank is discharged into the atmosphere without being adsorbed by the activated charcoal.
Naturally, however, the discharge of the fuel evaporative gas into the atmosphere constitutes no hindrance to the engine operation. Thus, a driver can hardly be aware of this fault as the fuel evaporative gas continues to be discharged into the atmosphere for a long period of time.